Cognitive dysfunction plays a role in a number of central nervous system disorders, including neurological disorders, such as Alzheimer's disease (AD), Parkinson disease and dementia, but also psychiatric disorders, such as schizophrenia, depression and bipolar disorders. As world population grows older the number of patients with dementia and AD is growing. Therefore, most people are familiar with the cognitive deficits related to these neurological diseases (Massoud and Gauthier, 2010).
However, also in psychiatric disorders cognitive impairment adversely affect the progress and the treatment outcome of the disease. A most prominent example is schizophrenia. Schizophrenia has a heterogeneous symptomatic picture (American Psychiatric Association, 1994) that may be divided into three distinct disease domains: positive symptoms (psychotic episodes of hallucinations, delusions and agitation), negative symptoms (social withdrawal, anhedonia, flattened affect) and cognitive deficits (deficits in executive function, verbal learning and memory, verbal fluency) (Thompson and Meltzer, 1993).
Whereas positive symptoms are essentially alleviated by dopamine D2 receptor antagonist and second generation antipsychotics negative symptoms and cognitive deficits are still hardly affected by current treatments. Therefore, research on cognitive deficits in schizophrenia has been intensified over the past years. A worldwide network initiative, called MATRICS, has been founded to characterise the cognitive deficits more deeply and to find novel therapies (Young et al., 2009).
However, cognitive impairment is also seen in patients with depression, bipolar disorders (Sachs et al., 2007; Pavuluri et al., 2009) and in many patients with disorders usually first diagnosed in infancy, childhood and adolescence, such as attention deficit/hyperactivity disorder (ADHD) (Jucaite et al., 2005; Turner et al., 2003).
Depression is a severe mental disorder which extremely impairs daily life. Its prevalence is about 10% of the world population with an incidence of 2% according to WHO. Women are more affected than men and elder people more than younger people. The disorder mostly implies a life-long treatment due to the progression of the disease and permanent total disability.
The most prominent symptoms of the disease are anhedonia, feeling of hopelessness, decreased self esteem, loss of appetite and sleep disturbances. Suicide ideation is also a common symptom of depression and about 10% of depressed patients (Holma et al., 2010) attempt suicide. Depression is often combined with anxiety disorders. Interestingly, it is less known that depression is also regularly associated with various cognitive impairments (Gualtieri et al., 2006; Mandelli et al., 2006). Here, deficits of attentional and executive functions are mostly reported (Paelecke-Habermann et al., 2005). Cognitive deficits are even discussed to be involved in the development of the disease (Beck depression model, Beck, 2008). More recent studies indicate that the severity of the cognitive deficits may predict non-response to certain antidepressant treatment (Dunkin et al., 2000; Gorlyn et al., 2008).
Up to now, current antidepressant therapy seems not to be sufficient regarding cognitive deficits. Elder antidepressants are reported to impair memory in animal models of learning and memory probably due to their anticholinergic component (Kumar and Kulkarni, 1996). In contrast, SSRIs, in particular fluoxetine, are described to impair hippocampal-independent but not hippocampal dependent learning in different rodent models (Valluzi and Chan, 2007). At least, in clinic current therapy it is not possible to fully reverse cognitive deficits. Thus, in depressive patients who had been successfully treated cognitive performance could be improved but not normalised (Gualtieri et al., 2006). Therefore, an antidepressant with higher efficacy on cognitive impairment may improve disease outcome.
Bipolar disorders are characterized by complex symptomatology, including severe symptoms of mood disorders but also manic episodes and cognitive deficits. The Diagnostic and Statistical Manual, 4th edition and International Classification of Mental Disorder recommend subgroups of bipolar disorder based on whether depressive or manic [psychotic] symptoms and episodes are dominating and on the frequency of the episodes (Gaiwani, 2009). Pharmacological agents commonly used in the management of bipolar disorder include lithium; anticonvulsants, such as valproate, carbamazepine and lamotrigine; and recent years have witnessed increasing use of atypical antipsychotics (Altamura et al., 2011). As a problem of current therapy the development of tolerance against anticonvulsant treatment and 30% of treatment refractory cases are described (Post and Weiss, 2010; Gaiwani, 2009).
Attention deficit hyperactivity disorder (ADHD) is a central nervous system disorder that is mainly defined by its clinical signs. ADHD shows a heterogeneous symptom pattern in humans. The most important indicators are attention deficits, impulsivity and a hyperactivity that is primarily seen in boys. The disease starts at an early age and symptoms are most intense during childhood. After puberty the signs of the disease are more masked and focus on cognitive dysfunction (Jucaite et al. 2005; Turner et al. 2003). Although modern research broadened the understanding of the pathomechanism the exact etiology of the disease remains unclear.
Interestingly, the symptoms seen in ADHD are not due to a hyperactivity but a hypoactivity of the so called executive loop of the striatum (Winstanley et al., 2006; Plizska, 2005). The executive loop is responsible for the regulation of cognitive processes such as planning, working memory and attention (Benke et al., 2003; Easton et al., 2007). A dysfunction of the prefrontal cortex or other pathways within the loop induces impulsivity and a loss of the ability to filter stimuli that come from the outside. The latter causes the symptoms of sustained attention and hyperactivity (Roberts and Wallis, 2000; Gonzales et al., 2000). The dopaminergic neurotransmitter system plays a central role in regulating the activity of the executive loop (Jucaite et al., 2005). This conclusion is also supported by the current treatment for ADHD that aims for an activation of the dopaminergic neurotransmitter system (Jucaite et al., 2005).
Phosphodiesterases (PDE) are expressed in nearly all mammalian cells. To date eleven families of phosphodiesterases have been identified in mammals (Essayan, 2001). It is well established that PDEs are critically involved in cell signalling. Specifically, PDEs are known to inactivate the cyclic nucleotides cAMP and/or cGMP (Soderling and Beavo, 2000). The cyclic nucleotides cAMP and cGMP are synthesised by the adenylyl and guanylyl cyclases and are second messengers that control many key cellular functions. The synthesis of cAMP and cGMP is regulated by different G-protein-coupled receptor types including dopamine D1 and D2 receptors (Mutschler, 2001).
The phosphodiesterases of the different families vary in their substrate selectivity. Thus, some families only hydrolyse cAMP others only cGMP. Some phosphodiesterases, such as phosphodiesterase 2 and 10, inactivate both cAMP and cGMP (Menniti et al., 2006).
Furthermore, there is a difference in the distribution of the different phosphodiesterases within the organism and additionally, within any particular tissue or organ. For instance, the distribution pattern of the phosphodiesterases within the brain is quite specific (Menniti et al., 2006).
Finally, phosphodiesterase families have different regulatory properties and intracellular location; some are bound to cell membranes and some are dissociated in the cytoplasm, additionally, a division into various intracellular compartments has been reported (Conti and Jin, 1999).
These differences in the function and location of the different PDE enzyme families suggest that the individual phosphodiesterases are selectively involved in regulating many different physiological processes. Accordingly, selective phosphodiesterase inhibitors may with fine specificity regulate different physiological and pathophysiological processes.
PDE2 and PDE10 hydrolyse both, cGMP and cAMP (Menniti et al., 2006; Soderling et al., 1999; Kotera et al., 1999).
They are both abundantly expressed in the brain indicating their relevance in CNS function (Bolger et al., 1994; Menniti et al., 2001).
PDE2 mRNA is mainly distributed in olfactory bulb, olfactory tubercle, cortex, amygdala, striatum, and hippocampus (Lakics et al., 2005; van Staveren et al., 2003). PDE10 (PDE10A) is primarily expressed in the nucleus accumbens and the caudate putamen. Areas with moderate expression are the thalamus, hippocampus, frontal cortex and olfactory tubercle (Menniti et al., 2001).
Although there are certainly fine differences in the function and expression patterns of PDE2 and 10, the expression of PDE2 in the hippocampus, the cortex and in the striatum and the expression of PDE10 in striatum, hippocampus and frontal cortex indicate an involvement in the mechanism of learning and memory/cognition. This is further supported by the fact that increased levels of both cGMP and cAMP are involved in the process of short and long term potentiation (LTP) forming (Blokland et al., 2006; Prickaerts et al., 2002). LTP is regarded as the electrophysiological basis of long term memory (Baddeley, 2003). Boess et al. (2004) showed that PDE2 inhibitors amplify the generation of LTP. Additionally, it is reported that the selective PDE2 inhibitor BAY60-7550 enhances learning and memory in rats and mice in different animal models (Boess et al., 2004; Rutten et al., 2006). Similar pro-cognitive effects are described for selective PDE10 inhibitors, such as papaverine and MP-10. Rodefer et al. (2005) have found that papaverine reverses attentional set-shifting deficits induced by subchronic administration of phencyclidine, an NMDA antagonist, in rats. Grauer et al. (2009) could show a positive effect of papaverine and MP-10 on cognitive deficits in the novel object recognition and in prepulse inhibition of acoustic startle response in rats. These data support the procognitive effect of PDE2 and/or 10 and a synergistic effect of PDE2 and 10 on cognition.
Furthermore, the expression of PDE2 in the nucleus accumbens (part of the striatum), the olfactory bulb, the olfactory tubercle and the amygdala and the expression of PDE10 in the nucleus accumbens, the olfactory tubercle and the thalamus supports additional involvement of PDE2 and 10 in the pathophysiology of anxiety and depression (Modell et al., 1990). This is supported by in vivo studies. The selective PDE2 inhibitors BAY60-7550 and ND-7001 are described to be effective in animal models of anxiety and stress-induced behavior (Masood et al., 2008, 2009).
In addition to the pro-cognitive and antidepressant potential of PDE10 inhibition there is evidence for an additional antipsychotic potential of PDE10 inhibitors. In the striatum PDE10 is predominately found postsynaptic in the medium spiny neurons (Xie et al., 2006). By this location, PDE10 may have an important influence on the signal cascade induced by dopaminergic and glutamatergic input on the striatum, two neurotransmitter systems playing a predominate role in the pathomechanism of psychosis. Focusing on the dopaminergic input on the medium spiny neurons, PDE10A inhibitors by up-regulating cAMP and cGMP levels act as D1 agonists and D2 antagonists because the activation of Gs-protein coupled dopamine D1 receptor increases intracellular cAMP, whereas the activation of the Gi-protein coupled dopamine D2 receptor decreases intracellular cAMP levels through inhibition of adenylyl cyclase activity (Mutschler et al., 2001). Accordingly, PDE10 inhibitors are reported to be active in several animal models of schizophrenia (Schmidt et al., 2008; Siuciak et al., 2006; Grauer et al., 2009).
PDE10 inhibitors have been disclosed recently in J. Med. Chem, 2011, 54, 7621-7638.
For drugs with an intended action in the central nervous system (CNS), it is assumed that unbound drug in interstitial spaces in the brain is in direct contact or in equilibrium with the site of action (de Lange and Danhof, 2002).
It is commonly accepted that unbound or free drug is the species available for interaction with drug targets within the body, and this is referred to as the free drug hypothesis.
Because cerebrospinal fluid (CSF) is in direct contact with the brain tissue, it is assumed to readily equilibrate with brain interstitial fluid concentration (Meineke et al., 2002; Shen et al., 2004) so that CSF concentration is used as a common surrogate measure for drug unbound concentration in clinical pharmacology studies (Bonati et al., 1982; Cherubin et al., 1989; Garver, 1989; Reiter and Doron, 1996; Ostermann et al., 2004). Accordingly, for compounds with an intended action in the central nervous system it is important that they reach a high CSF concentration and a high CSF to plasma ratio in order to have high pharmacological activity in the CNS.
Inhibition of the hERG channel by xenobiotics and subsequent delayed cardiac repolarization is associated with an increased risk for a specific polymorphic ventricular tachyarrhythmia, torsade de pointes, as established by Sanguinetti et al. (1995, Cell, April 21, 81(2):299-307) and a large body of subsequent evidence. As such, low hERG channel inhibition, such as that shown by the compounds of the present invention, is highly desirable for therapeutics.
Several families of PDE2 inhibitors are known. Imidazotriazinones are claimed in WO 2002/068423 for the treatment of e.g. memory deficiency, cognitive disorders, dementia and Alzheimer's disease. Oxindoles are described in WO 2005/041957 for the treatment of dementia. Further inhibitors of PDE2 are known from WO 2007/121319 for the treatment of anxiety and depression, from WO 2013/034761, WO 2012/104293 and WO2013/000924 for the treatment of neurological and psychiatric disorders, from WO 2006/072615, WO 2006/072612, WO 2006/024640 and WO 2005/113517 for the treatment of arthritis, cancer, edema and septic shock, from WO 2005/063723 for the treatment of renal and liver failure, liver dysfunction, restless leg syndrome, rheumatic disorders, arthritis, rhinitis, asthma and obesity, from WO 2005/041957 for the treatment of cancer and thrombotic disorders, from WO 2006/102728 for the treatment of angina pectoris and hypertension from WO 2008/043461 for the treatment of cardiovascular disorders, erectile dysfunction, inflammation and renal failure and from WO 2005/061497 for the treatment of e.g. dementia, memory disorders, cancer and osteoporosis.
Finally, benzodiazepines are described in WO 2005/063723 for the general treatment of CNS diseases including anxiety, depression, ADHD, neurodegeneration, Alzheimer's disease and psychosis.